Try pressing the Auto adjust button on your Monitor when booting the xbox. If it's got one it should calibrate the screen to sync with the settings the console needs
is it just one game that appears blurry? or every game? if you were having major problems then you would have black and white screen with every game you played, but as you don't mention that I think you may want to tinker with the xbox console / display settings a bit more. Don't forget to recalibrate the monitor after each time you change the setting, just so the screen can refresh and display the latest changes made.